1. Field of the Invention
The present invention relates to amplifiers having a biased current input and arranged to amplify a varying input signal. The invention is particularly but not exclusively concerned with transconductance amplifiers in envelope tracking architectures.
2. Description of the Related Art
Frequency domain duplex (FDD) systems include transceivers that have a transmitter and a receiver which operate at different carrier frequencies. A simple exemplary architecture of such an FDD system is illustrated in FIG. 1. A transceiver includes a transmitter block 6 which receives an input signal on line 2 to be transmitted by an antenna 18. A receiver block 8 receives signals that are detected at the antenna 18 and delivers them on signal line 4. The output of the transmitter 6 is delivered to a duplex filter 14 on line 10. The received signal from the antenna 18 is delivered from the duplex filter 14 on a line 12 to the receiver 8. The antenna 18 is connected to the duplex filter 14 via a line 16.
In such an FDD system it is essential that energy from the transmitter does not block the receiver. This may occur because the duplex filter on the output of the transmitter has only limited attenuation. Any noise present at the receiver frequency on the transmitter output due to noise from the transmitter has the potential to cause receiver blocking.
The transmitter circuitry 6 typically includes a transconductance amplifier. A transconductance amplifier generates a current which is proportional to its input voltage. Any noise present at the receiver frequency on the transmitter output due to noise from a transconductance amplifier in the transmitter has the potential to cause the above-mentioned receiver blocking. Any such noise must preferably not be allowed to exceed a low level while the power consumption of the transconductance amplifier is minimised. Thus efficient, low noise, high linearity transconductance amplifiers are required in FDD systems.
A typical transconductance amplifier uses a class A or continuous bias scheme that is independent of the input signal level. In a transmitter incorporating an envelope tracking modulated power supply, a typical transconductance amplifier uses a class A or continuous bias scheme that is independent of the modulation envelope.
With such a bias scheme, the transconductance amplifier consumes the same power and generates the same noise at the signal troughs as it does at the signal peaks.
It is an aim of the invention to control an amplifier, such as a transconductance amplifier, to reduce generated noise and/or reduce consumed power.